Ultraviolet (UV) radiometers have been developed to measure the power of beams of UV radiation. Such UV beams are typically produced by lasers, and the power of UV laser beams is measured so that calibration of the laser is assured and adjusted if required. If the power of a laser is not accurately measured, the laser may not perform as desired in delicate or precision uses.
In a UV radiometer shown and described in U.S. Pat. No. 4,885,471, the disclosure of which is incorporated herein by reference, a beam of UV radiation is incident on a plate of material that converts the UV radiation to visible fluorescent radiation in response to and in proportion to the power of the beam of UV radiation. Various methods and apparatus are shown for collecting and measuring the fluorescent radiation produced in the plate, to indirectly measure the power of the incident beam of UV radiation. The basic technique is to position photodiodes or other photocells for receiving the fluorescent radiation produced in the converting plate, and using the photocell to produce an electrical signal proportionate to the amount of fluorescent radiation that has been collected by the photocell or photocells.
The converting plate is generally a crystal containing fluorescent media, often referred to as “impurities” because they are undesirable for most uses of the crystals. However, even though they are called “impurities”, they are both intended and necessary to the measurement of the power of a beam of UV radiation by the aforementioned fluorescing technique. One such material is an undoped oxide crystal and rare earth doped crystals will also suffice. Another material is a rare earth doped garnet, such as Ce3+:Y3AL5O12(YAG). Besides cerium (Ce), other rare earth elements suitable for doping include neodymium (Nd), lanthanum (La) and Europium (Eu), as well as others.
The crystals are grown as a boule, and the methods of growing crystals is such that the concentration of the fluorescent impurities generally varies along the direction of growth. Crystal converting plates are made by first cutting a cylindrical core of the boule, taken across the growth direction. Circular converting plates are then sliced from the cylindrical core, so that the converting plates will typically have impurity concentration gradients generally along the growth direction of the boule, which is across the diameter of the converting plate, and when used for measurement of the strength of a beam of UV radiation, converting plates cut from these crystals will have response variations generally across the growth direction and, to a lesser extent, along other directions taken across the crystal plate. It should be noted that the response gradients are not always or exactly aligned in the growth direction, depending on the conditions of the crystal growth and other variables. These response variations can be as large as 20% across the diameter of a crystal converting plate, and this has proven unacceptable for measuring the power of an incident UV beam because the measurement becomes dependent on where the UV beam is incident on the converting plate.
Although the response variations can be minimized by cutting the crystal converting plates perpendicularly to the growth direction of the boule, this does not always eliminate the variations in response, and some UV detection applications require specific orientations that are difficult or perhaps impossible to grow along the desired axis.
The net result is that UV radiometers generally do not produce a uniform measurement when a beam of incident UV radiation is varied in its point of incidence on a crystal converting plate of a UV radiometer, wherein the measurement achieved by the UV radiometer is a function of both the strength of the beam and the non-uniform response of the crystal converting plate. This is, of course, not desirable and improvement would be a valuable advance in the art.